The present invention relates to an optical pickup, and more particularly, to an optical pickup adopting a multi-coated polarization holographic optical element (HOE).
Generally, an optical pickup is for recording and/or reproducing information related to pictures, sound and data on an optical recording medium in a noncontact manner.
FIG. 1 is a schematic perspective view of a conventional optical pickup adopting a general HOE. The optical pickup includes a light source 20, an HOE 50 for changing a traveling path of incident light, a phase delay plate 60 for changing a polarization direction of the incident light, an objective lens 70 for converging the light entered from the light source 20 on a recording surface of a recording medium 10, and a photodetector 80 for detecting an information signal and an error signal.
The objective lens 70 is driven by an actuator (not shown) to correct tracking and focusing errors based on the error signal detected from the photodetector 80.
The HOE 50 is arranged at a position along an optical path between the objective lens 70 and the light source 20, and is formed by etching a substrate to form its hologram pattern to provide different diffraction characteristics in accordance with the polarization direction of the incident light. Generally, the phase delay plate 60 includes a .lambda./4 waveform plate for changing a linearly polarized light incident from the light source 20 into a circularly polarized light and changing the circularly polarized light reflected from the recording medium into the linearly polarized light. A reflecting mirror 40 for changing a traveling path of the light emitted from the light source 20 is arranged at an angle between the light source 20 and the HOE 50 under the consideration of the optical arrangement of the optical pickup. Also, a collimating lens 30 for collimating a divergent light emitted from the light source 20 may be included in the optical path between the light source 20 and the reflecting mirror 40.
As described above, the optical pickup adopting the HOE 50 has a simpler optical structure than an optical pickup adopting a beam splitter.
As shown in FIG. 2, the conventional HOE 50 adopted in the above optical pickup uses a substrate 51 made of LiNb.sub.3 having a high refractive index, wherein a plurality of grooves 52 are formed by an etching process. The light incident on the HOE 50 is transmitted in a region having the grooves 52 without a phase delay. Meanwhile, the phase of a P-polarized light component or an S-polarized light component is reversed by 180.degree. in a region without the grooves 52. Accordingly, a polarized light component passed through the HOE 50, such as a light 54 of a P-polarized light component, is transmitted directly, while the other polarized light component such as a light 55 of an S-polarized light component is diffractively transmitted. The HOE 50 can transmit the light directly or diffractively according to the polarized light component of the incident light. However, the material of the substrate used therefor, i.e., LiNb.sub.3, is expensive, and thus, manufacturing costs of the conventional optical pickup adopting the HOE are quite high.